My freelancer/contractor habits formed when I was in my early 20s. Initially I found I was a 'collector' - I liked having lots of open jobs since it felt good to be needed. About 18 months into doing this I realized I wasn't getting anything finished for anyone, so I started to cut back. Overcommitment is the most common problem, and it is usually due to limited understanding of one's own capacities.
I've found that I can handle two clients, and perhaps a third in reserve, with the general understanding that the first client is 'core' and I do most of my billable work for that client. The second client is a lower priority, generally with limited budget, limited objectives, 'maintenance mode', etc. where I can work on their stuff to keep them running but it isn't major development. The third client, if any, is 'landed' - they are committed to doing the project, but they are aware that I can't really pick up their work until the project I'm working on now shifts from 'major effort' to 'maintenance'. I won't solicit work for a new major effort if the one I'm currently doing is expected to last more than six months.
If one is interviewing a freelancer, one would have to find out 'where they are' and 'where you are' - if they have a major project and you're asking them to do a major project you're out of luck. If they have one client and you want them to do maintenance the two of you have compatible objectives.
Unprofessional behavior includes things like stringing the client along - 'yeah I'm working on it I'll have something for you at the end of the week' - this for six months straight. If they aren't talking to you about current and emerging requirements they probably aren't writing any code - in any project I've ever done I had to get clarifications from the customer every few days. If you have a remote login arrangement and you don't see any activity for an unusual amount of time they're not paying any attention to your project.
What you want to get is a perspective on is how many open bookings the freelancer currently has. If they have more than two 'major projects' don't proceed further. If they have a single project, see if it's likely that the client will ask for the 'next phase' once this one is done. A point to remember is that an existing client is 'the devil you know' - they've set up their billing and spending habits around the client's payment performance - if yours is different it would have to be 'better'. If the existing client wants the freelancer to start a new phase this project might displace yours.
Some freelancers have a 'lot of little' things going on and no large scale project. I would run into someone what would spend months nailing down six week long jobs. 'Major' development projects normally last around 18 months, so a contractor should have one of these, and this will obviously involve a budget in the $100,000 range. If you're asking someone to do a two week job, and that's similar to most of the other stuff they're doing, they're spending most of their time out getting business instead of doing billable work. If they have had 'large' projects in the recent past, this is OK - they will most likely pick up another which will diminish their distractions. If there doesn't seem to be any prospects for the big job on the horizon, your project is prioritized in relation to the marketing effort rather than the production effort.